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Re: [Xen-devel] Debug-Registers in HVM domain destroyed

  • To: Jan Beulich <JBeulich@xxxxxxxx>
  • From: Juergen Gross <juergen.gross@xxxxxxxxxxxxxx>
  • Date: Thu, 20 Feb 2014 08:44:51 +0100
  • Cc: xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
  • Delivery-date: Thu, 20 Feb 2014 07:45:18 +0000
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On 18.02.2014 13:56, Jan Beulich wrote:
On 18.02.14 at 13:48, Juergen Gross <juergen.gross@xxxxxxxxxxxxxx> wrote:
On 14.02.2014 14:02, Jan Beulich wrote:
On 14.02.14 at 13:18, Juergen Gross <juergen.gross@xxxxxxxxxxxxxx> wrote:
Is this the case when the guest itself uses single stepping? Initially the
debug trap shouldn't cause a VMEXIT, I think.

That looks like a bug, indeed - it's missing from the initially set
exception_bitmap. Could you check whether adding this in
construct_vmcs() addresses that part of the issue? (A proper fix
would likely include further adjustments to the setting of this flag,
e.g. clearing it alongside clearing the DR intercept.) But then
again all of this already depends on cpu_has_monitor_trap_flag -
if that's set on your system, maybe you could try suppressing its
detection (by removing CPU_BASED_MONITOR_TRAP_FLAG from
the optional feature set in vmx_init_vmcs_config())?

I've currently a test running with the attached patch (the bug was hit about
once every 3 hours, test is running now for about 4 hours without problem).
Test machine is running with Xen 4.2.3 hypervisor from SLES11 SP3.

Which, if it continues running fine, would confirm the theory.
I'd like to defer to the VMX folks though for putting together a
proper fix then - I'd likely overlook some corner case.

Okay, theory confirmed.

Unless you want to do it, I'll start another thread with the info found so far
and include Jun Nakajima and Eddie Dong.


Juergen Gross                 Principal Developer Operating Systems
PBG PDG ES&S SWE OS6                   Telephone: +49 (0) 89 62060 2932
Fujitsu                                   e-mail: juergen.gross@xxxxxxxxxxxxxx
Mies-van-der-Rohe-Str. 8                Internet: ts.fujitsu.com
D-80807 Muenchen                 Company details: ts.fujitsu.com/imprint.html

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